swift234

Teach ASTWalker to walk some more implicit expressions and
declarations generated by the type-checker.

Unfortunately, it's not possible to directly check this
because the verifier is itself based on ASTWalker.  But I'm
going to add an assertion to SILGen in a follow-up that
assumes that TypeCheckError has visited every ApplyExpr,
and since SILGen already walks most of the AST with its
own custom walker, that will check this well enough.

Swift SVN r28732